The Automatic Identification System (AIS) communication system was developed to provide identification and location information to sea vessels and shore stations with the aim of exchanging different type of data including position, identification, course, speed and others. On the one hand this allows vessels to anticipate and thus avoid collisions in the sea by means of a continuous traffic monitoring with several navigation aids; on the other hand AIS also offers important ship monitoring services to coastal guards or search and rescue organizations.
Each ship equipped with an AIS apparatus broadcasts information (data) in small slots of 26.67 ms. In each of these slots a message of 256 bits is transmitted at a rate of 9600 b/s using a binary GMSK (Gaussian Minimum Shift Keying) modulation over two VHF carriers. Nearby AIS emitters synchronize with each other in order to avoid packet collisions, i.e. emission of more than one packet in a same time slot by different emitters (time slots are defined globally on the basis of a common temporal reference provided by GPS). As a result Self Organized Time Division Multiplex Access (SOTDMA) regions are formed. Each SOTDMA region is designed to cope with path delays not longer than 12 bits, which translates into a maximum range of about 200 nautical miles, but typically the radio frequency coverage is limited to about 40 nautical miles. Within this range all the ships in visibility transmit with the SOTDMA protocol which ensures that collisions are prevented from bursts transmitted by different ships.
Recently, the interest of detecting and tracking ships at distances from coastlines that are larger than can be accomplished by normal terrestrial VHF communications has grown. Requirements of these long range applications such as better handling of hazardous cargo, improved security and countering illegal operations suggested a need to detect ships at very long distanced from shores.
Satellite based AIS is presented as a promising solution to overcome the terrestrial VHF coverage limitation with the potential to provide AIS detection service coverage on any given area on the Earth. In particular, a LEO (low earth orbit) constellation of small-size satellites, with an altitude ranging from 600 to 1000 km, could provide global coverage. Each satellite would be provided with an on-board small VHF antenna with a field of view spanning over a few thousands of nautical miles and comprising up to several hundreds of SOTDMA cells.
A number of projects, both of public and private initiative, are currently undergoing to analyze the concept of satellite reception of AIS signals. Some trials have also been carried out. Overall it has been proved than the new system is feasible provided efficient receiver techniques are utilized. Satellite-based AIS, however, has to face with additional technical challenges that were not considered in the original AIS standard:                AIS messages from ships belonging to different SOTDMA cells are not synchronized and therefore can collide;        Satellite motion with respect to the emitters induces a significant Doppler shift of the carrier frequency;        Signal to noise ratio (Eb/N0) is lower than in terrestrial AIS; and        Relative propagation channel delay among the population of ships in visibility at any given time is much higher than in terrestrial AIS.        
Document [R1] describes a space-borne AIS receiver implemented on a FPGA, based on a conventional 2-bits differential demodulator. This receiver has the advantage of simplicity, but its performances in terms of BER (bit error rate) or PER (packet error rate) as a function of signal-to-noise ratio (Eb/N0) and its sensitivity to co-channel interference are not satisfactory. Performances get even worst if imperfect carrier frequency recovery and timing synchronization are considered.
Document WO 2007/143478 describes a space-based network for detection and monitoring of global maritime shipping using AIS. This document does not disclose in detail any specific AIS receiving architecture. However it suggests using Doppler frequency difference and interference cancellation to deal with the problem of colliding messages from uncoordinated AIS emitters.